Long term project?
That does not actually mean anything.
If a client hires you for a one-hour job, she might like your work so much she keeps you busy and well-paid for many years. If she hires you for a long term project, she might decide tomorrow to sail around the world instead of continue working on a website with you. So there goes that.
Applying to the job is just the beginning of a conversation. It does not commit you to anything in particular.
Wina W wrote:
If that’s the case, how should freelancers bid in the proposal? Can we change the milestones and bid later when the job is already been accepted?
The contract price isn't set until the client makes (and you accept) an offer. You can submit your proposal based on assumed number of illustrations (or books) that you specify in your proposal, and if in the course of interviewing you find out the scope is larger than you expected you can request a higher price.
If the client insists on offering a price that you believe is unfair, you have the option to decline.